londong explorer: more expensive in May?

Hi! I just wanted to ask why London explorer is more expensive in May? Is it because of the month? or will the price still decrease? Prices for feb and march are much cheaper. <img src="http://connect.contiki.com/groupee_common/emoticons/icon_smile.gif" alt="Smiler" width="15" height="15"><!--graemlin::)--><BR><BR>Thanks!!